1. Field of the Invention
The present invention relates to a detachable optical fiber connector and an endoscope system using this connector, and more specifically relates to the optical fiber connector that is suitable for transmission of high power laser light and the endoscope system using the same.
2. Description Related to the Prior Art
Physical contact connection (hereinafter called PC connection) is known as a method for connecting optical fibers, in which ends of the optical fibers tightly make contact with each other. In the PC connection, an end portion of each optical fiber is inserted into and fixed in a cylindrical ferrule, and an end face of each optical fiber is polished into a convex spherical shape together with an end face of the ferrule. The ferrules with the optical fibers are inserted into both ends of a cylindrical sleeve, so that the polished end faces of the ferrules make contact with each other in the sleeve. Thus, the optical fibers exposed from the end faces of the ferrules directly and tightly make contact with each other in the sleeve.
In an endoscope system for imaging the inside of a human body, a fiber bundle, which is composed of a plurality of optical fibers tied together, is used as a light guide to transmit light from a light source unit to the inside of the body. The light guide is routed through a flexible insert section of an endoscope, which is introduced into the human body, and a universal cord for connecting the endoscope to the light source unit. An end of the light guide is connected to a lighting window provided in a distal end of the insert section. The other end of the light guide is connected to the light source unit via a connector provided on an end of the universal cord.
Since the connector is often detached from the light source unit, a tip of the connector tends to be dusty. Besides, the often detachment can cause damage to the end face of the light guide provided in the connector. If a connection section of the optical fibers becomes dusty or flawed, a connection loss of light transmitted through the optical fibers is increased. In addition, in a case where the end of the optical fiber has high optical power density, a dust particle or the flaw can catch fire, and the end of the optical fiber and the end of the ferrule can be burnt. Otherwise, the fire can spread over the optical fiber by a fiber fuse phenomenon.
To prevent the burn of the optical fiber due to the dust particle or the like, there is proposed an optical fiber connector that has low optical power density at the connection section. According to U.S. Pat. Nos. 7,333,702 and 6,542,665, for example, a graded-index fiber that functions as a collimator lens is fusion spliced to an end of a single-mode fiber, in order to enlarge a mode field diameter.
By the way, it is considered to use a laser light source unit in the endoscope system. Laser light with a short wavelength and high power outputted from the laser light source unit is guided through the light guide to a phosphor provided at the distal end of the insert section. The laser light excites the phosphor to obtain illumination light. In such a light guide for transmitting the laser light, a single strand of multimode fiber having a core diameter of, for example, 100 μm or more is used, instead of the optical fiber bundle. The use of the multimode fiber contributes reduction in the diameter of the insert section.
It is known that a phenomenon called dust collection effect occurs at a light transmission section with high optical power density, when the optical fiber transmits the high power laser light. The dust collection effect is a phenomenon in which the laser light photochemically reacts with vaporized organic substances and forms other substances, and the formed substances are deposited. The multimode fiber has a larger core diameter than a general single-mode fiber. Thus, it is difficult for the multimode fibers to precisely contact end faces of cores with each other by the PC connection. Consequently, apart of the end face of the core with high optical power density is exposed to the air, and the dust collection effect tends to occur there. The dust collection effect occurring in the optical fiber causes increase in the connection loss, as with the dust particle, and can result in the burn of the optical fiber or the fiber fuse phenomenon.
When the connected optical fibers transmit the high power laser light, there is a case where oxide (such as quartz and SIO2) contained in the optical fibers reacts with the laser light, and ends of the optical fibers adhere to each other in the PC connection section. It is known that this adhesion phenomenon is likely to occur, when the ends of the optical fibers or the ends of the ferrules are connected by the PC connection after UV cleaning. If the optical fibers adhere to each other in the connector of the endoscope system, the ends of the optical fibers will break upon detaching the connector from the laser light source unit. If the break occurs, the ends of the optical fibers require re-polishing with the ferrules, and hence repair becomes a big deal.
The adverse phenomena including the dust collection effect and the adhesion can be prevented by reducing the optical power density at the PC connection section with use of the graded-index fibers, as described in the U.S. Pat. Nos. 7,333,702 and 6,542,665. These prior arts, however, are intended for the single-mode fibers that transmit light of a long wavelength for communications. Accordingly, the optical fibers are different from those used in the endoscope system in type, a core diameter and the like, and hence the prior arts are not applicable to the optical fibers of the endoscope system.
Furthermore, light incident upon the graded-index fiber (GI fiber) is transmitted therethrough with traveling an optical path of a sine wave. The sine wave has a specific period in accordance with a refractive index profile of the GI fiber. When one pitch refers to one period of the sine wave, the GI fiber used as the collimator lens has to have a length of ¼×(2n−1) (n=1, 2, . . . ) pitch. However, since both ends of the GI fiber is polished before connection to the optical fiber, the GI fiber is difficult to precisely process into a desired length and this causes a low yield rate. Placing importance on cost, if the GI fibers of various lengths are used, collimation properties become unstable from product to product. In addition, even if the GI fiber can be processed into the desired length, variations in a refraction index in a longitudinal direction may cause poor collimation properties of the connector.